MINI REVIEW published: 11 November 2016 doi: 10.3389/fcell.2016.00127

Septins and Bacterial Infection

Vincenzo Torraca and Serge Mostowy *

Department of Medicine, MRC Centre of Molecular Bacteriology and Infection, Imperial College London, London, UK

Septins, a unique cytoskeletal component associated with cellular membranes, are increasingly recognized as having important roles in host defense against bacterial infection. A role for septins during invasion of Listeria monocytogenes into host cells was first proposed in 2002. Since then, work has shown that septins assemble in response to a wide variety of invasive bacterial pathogens, and septin assemblies can have different roles during the bacterial infection process. Here we review the interplay between septins and bacterial pathogens, highlighting septins as a structural determinant of host defense. We also discuss how investigation of septin assembly in response to bacterial infection can yield insight into basic cellular processes including phagocytosis, autophagy, and mitochondrial dynamics.

Keywords: actin, autophagy, cell-autonomous immunity, cytoskeleton, mitochondria, Listeria, Shigella, septins

INTRODUCTION

Work has shown that components of the cytoskeleton occupy a central role in innate immunity by promoting bacterial sensing and executing antibacterial functions (Mostowy, 2014; Mostowy Edited by: and Shenoy, 2015). On the other hand, several intracellular pathogens can exploit the host Manoj B. Menon, Hannover Medical School, Germany cytoskeleton for their own advantage to promote invasion, establish a replicative niche, and/or enable dissemination. During infection, some bacteria can invade non-phagocytic cells including Reviewed by: epithelial and endothelial cells (Cossart and Sansonetti, 2004; Haglund and Welch, 2011). After Kenneth G. Campellone, University of Connecticut, USA entry these pathogens remain enclosed within a membrane-bound compartment or escape to Klaus Aktories, the host cell cytosol (Fredlund and Enninga, 2014). The compartmentalized or cytosolic lifestyle University of Freiburg, Germany of intracellular pathogens can trigger rearrangements of the cytoskeleton and determine host *Correspondence: response to infection. For example, some bacteria that rupture the phagocytic vacuole and escape Serge Mostowy to the cytosol directly interact with components of the host cytoskeleton to initiate actin-based [email protected] motility for cell-to-cell spread (Welch and Way, 2013). To counteract bacterial pathogenesis, the cytoskeleton can mediate a variety of cell-autonomous immune defenses, such as activation of the Specialty section: inflammasome (a molecular platform processing inflammatory cytokines) or targeting pathogens This article was submitted to to autophagy (a cytosolic degradation process) (Mostowy and Shenoy, 2015). Signaling, In comparison to actin, relatively little is known about the role of septins during bacterial a section of the journal infection. Septins are GTP-binding proteins that associate with cellular membrane to form Frontiers in Cell and Developmental Biology filaments and ring-like structures (Mostowy and Cossart, 2012). They also interact with actin filaments and microtubules, and are therefore considered a cytoskeletal component. Septins were Received: 31 August 2016 discovered in Saccharomyces cerevisiae as crucial for cell division (Hartwell, 1971). Though septins Accepted: 26 October 2016 Published: 11 November 2016 are highly conserved in fungi and animals, their number across eukaryotic species is variable. There are 7 septins in S. cerevisiae, 2 in Caenorhabditis elegans, 5 in Drosophila melanogaster, at Citation: Torraca V and Mostowy S (2016) least 17 in zebrafish (Danio rerio), and 13 in both mice and humans (SEPT1-12 and SEPT14). Septins and Bacterial Infection. Vertebrate septins can be classified into 4 subgroups, namely the SEPT2, SEPT3, SEPT6, and SEPT7 Front. Cell Dev. Biol. 4:127. subgroups, based on homology of sequence and protein domains (Mostowy and Cossart, 2012) doi: 10.3389/fcell.2016.00127 (Figure 1A). Septin subunits from the different subgroups interact through their G (consisting

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FIGURE 1 | Structure and assembly of the septin cytoskeleton. (A) Prototypical structure of mammalian septins. Humans septins (SEPT1–SEPT12, SEPT14) are classified into four subgroups (SEPT2, SEPT3, SEPT6, SEPT7), and consist of three conserved domains: a phosphoinositide-binding polybasic region, a GTP-binding domain, and a septin unique element (SUE). Septins typically have a proline-rich motif at the amino-terminal and a coiled-coil domain at the carboxy-terminal, although the length of these regions varies. By interacting via their G and NC interfaces, septin monomers (B) from different subgroups (shown as different shades of red), form complexes (rods of 32–40 nm in length) that contain six (C) or eight (not shown) symmetrically arranged septins (2 monomers from each septin subgroup). When complexes are assembled end-to-end they form non-polar filaments (D). Septin filaments can associate laterally and form bundles (E), bundles of septin filaments can form higher-order structures, such as rings which are ∼0.6 µm in diameter (F). Adapted from Mostowy and Cossart (2012).

of the GTP-binding domain) and NC (consisting of the amino- support bacterial adherence to the host cell and enable bacterial and carboxy-terminal regions) interfaces, forming complexes internalization into both immune and non-immune cells. that can join end-to-end to form filaments. Septin filaments can associate, bundle, and go on to form higher-order structures such as rings (Mostowy and Cossart, 2012)(Figures 1B–F). Formation of Pedestal-Like Structures Septins are involved in numerous biological processes and during Enteropathogenic Escherichia coli have been implicated in a wide variety of pathological conditions Infection including cancer, neurodegenerative disorders, and infection. Enteropathogenic E. coli (EPEC) is a Gram-negative bacterium This protein family was first studied in the context of bacterial responsible for diarrheal disease in humans. By using a type III infection 15 years ago (Pizarro-Cerdá et al., 2002). Identified by secretion system (T3SS), a molecular syringe that secretes effector mass spectrometry, SEPT9 was associated with the invasion of proteins into the host cell, EPEC injects Translocated Intimin Listeria monocytogenes into epithelial cells. Since then, septins receptor (Tir) into the plasma membrane so that it functions as have been associated with a variety of bacterial pathogens and an anchor for Intimin, an effector protein located on the EPEC different stages of the host cell infection process (Mostowy outer membrane (Croxen et al., 2013). Tir-Intimin interactions and Cossart, 2012; Krokowski and Mostowy, 2016). Here, we mediate the recruitment of Neural Wiskott-Aldrich Syndrome review the literature implicating septins in bacterial pathogenesis Protein (N-WASP) and the Actin-Related Protein 2/3 (ARP2/3) (Supplementary Table S1), and discuss how investigation complex, remodeling of cortical actin filaments, flattening of of septin-bacteria interplay can provide novel insights into intestinal microvilli, and ultimately the formation of pedestal- fundamental processes underlying bacterial infection and also like structures raising the bacteria above the plasma membrane host cell physiology. (Croxen et al., 2013; Lai et al., 2013). A recent study discovered that EPEC infection phosphorylates SEPT9 in a T3SS-dependent manner (Scholz et al., 2015). The SEPTIN FUNCTION IN BACTERIAL depletion or impaired phosphorylation of SEPT9 reduced ADHESION AND ENTRY INTO HOST CELLS adherence of EPEC to the host cells and also EPEC-mediated cytotoxicity. How septins and their phosphorylation contribute Bacterial adhesion to the host cell is fundamental for to EPEC adhesion remains to be established. Work has shown some pathogens to establish infection. Adhesion prevents an important role for septin phosphorylation in higher-order the mechanical clearance of extracellular bacteria (e.g., assembly into rings (Dobbelaere et al., 2003; Kinoshita, 2003; enteropathogenic Escherichia coli and Clostridium difficile) Hernández-Rodríguez and Momany, 2012; Meseroll et al., and facilitates the host cell entry of invasive bacteria (e.g., L. 2013). Given that septins are involved in actin dynamics, septin monocytogenes, Yersinia spp., Shigella flexneri, and Salmonella assembly may promote actin rearrangements and membrane spp.). In this section, we summarize how several bacterial dynamics which mediate pedestal biogenesis (Scholz et al., pathogens manipulate septins at the plasma membrane to 2015) (Figure 2A). The T3SS effector that mediates SEPT9

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FIGURE 2 | Septin recruitment and function in bacterial infections. (A) The enteropathogenic E. coli (EPEC) pedestal. Type 3 secretion system (T3SS) effectors of EPEC are implicated in septin phosphorylation and assembly into filaments and rings. It is proposed that septins assemblies (depicted here as rings) remodel cortical actin for pedestal biogenesis (B) The C. difficile microtubule-based protrusion. Intoxication with the C. difficile toxin (CDT) leads to patches of cortical actin/septin depolymerization followed by rearrangement of septins into ring-like structures, facilitated by the factors Cell Division Control Protein 42 (CDC42) and Binder of Rho GTPase (BORG). By interacting with End Binding 1 (EB1) protein, septins redirect microtubule polymerization and initiate protrusion formation. Protrusions prevent the mechanical clearance of C. difficile by wrapping around the bacterium. (C) Zipper-mediated entry by L. monocytogenes. During invasion of Listeria into host cells, interaction of the virulence factor Internalin B (InlB) with its membrane receptor Met, initiates septin recruitment to the entry site where it forms ring-like structures at the base of the phagocytic cup. (D) Trigger-mediated entry by S. flexneri or S. Typhimurium. Septin ring-like structures associate with the bacteria upon injection of T3SS effectors into the host cell to induce membrane ruffles and macropinocytosis at the bacterial entry site. In the case of (C,D), septins at the plasma membrane may facilitate actin remodeling, membrane protrusion, and bacterial internalization by acting as diffusion barriers for lipids and signaling molecules. (E) The C. trachomatis inclusion vacuole. Septin and actin filaments form a coat that associate with the cytosolic surface of the inclusion vacuole, and contributes to its remodeling to accommodate bacterial growth. The chlamydial enzyme CPAF is able to cleave septins for control of inclusion remodeling. (F) The L. monocytogenes or S. flexneri actin tail. Although dispensable for actin-based motility, septins assemble into rings around a subset of cytosolic bacteria polymerizing actin tails. The precise role of septins in the functionality of the actin tail remains unknown. (G) The S. flexneri septin cage. Septins assemble into cage-like structures around a subset of cytosolic bacteria polymerizing actin. In the case of Shigella, septin cage assembly is promoted by mitochondria and leads to the restriction of bacterial replication by autophagy. phosphorylation remains unknown, however a previous study coordinate rearrangements of the actin cytoskeleton and plasma performed in S. cerevisiae showed that overexpression of the membrane. T3SS effector proteins Mitochondrial associated protein (Map) and E. coli secretion protein F (EspF) cause abnormal and Microtubule-Based Protrusions from mislocalized septin assemblies that affect morphogenesis and Clostridium difficile Infection cell division (Rodríguez-Escudero et al., 2005). The same study Hypervirulent strains of C. difficile infect humans provoking implicated Map and EspF in the activation of a phosphorylation colitis, antibiotic-associated diarrhea, and occasionally death. cascade via the Mitogen-Activated Protein Kinase (MAPK) The increased virulence of some C. difficile isolates is associated pathway. Taken together, it is tempting to speculate that with the expression of the C. difficile transferase (CDT), a EPEC virulence factors activate a phosphorylation cascade to toxin that mediates formation of microtubule-based protrusions, mediate the recruitment and assembly of septins at the plasma which wrap around bacteria and increase their adherence to the membrane for actin-mediated pedestal formation. Further host cell (Schwan et al., 2014). When exposing cells to CDT, experiments will be required to address this hypothesis. Notably, the cellular cortex undergoes extensive remodeling, resulting increased phosphorylation of SEPT9 has also been identified in patches of actin depolymerization. Complexes of SEPT2- from infections with other T3SS-positive enterobacteria, SEPT6-SEPT7-SEPT9 are recruited to the plasma membrane including Shigella and Salmonella spp. (Rogers et al., 2011; shortly after initial depolymerization events and form collar- Schmutz et al., 2013), raising the possibility that septin like structures that initiate CDT-dependent protrusions (Nölke phosphorylation is commonly exploited by bacterial pathogens to et al., 2016)(Figure 2B). Depletion of septins by siRNA or their

Frontiers in Cell and Developmental Biology | www.frontiersin.org 3 November 2016 | Volume 4 | Article 127 Torraca and Mostowy Septin-Bacteria Interplay inhibition with forchlorfenuron reduced protrusion formation, et al., 2009b) (Figure 2D). Here, septins may additionally while septin overexpression increased protrusion emergence. facilitate membrane fusion events during the bacteria-induced At the protrusion base, septins interact with the GTPase Cell macropinosome trafficking (Dolat and Spiliotis, 2016; Weiner Division Control Protein 42 (CDC42) and its effector Binder et al., 2016). Together, this highlights septin assembly as a general of Rho GTPase (BORG) (Nölke et al., 2016). These results are response to actin-mediated bacterial invasion. In addition to consistent with previous work using non-infected HeLa cells the above studies performed using infection of non-phagocytic showing the regulation of septin assembly by BORGs (Joberty host cells, work has shown that SEPT2 and SEPT11 assemble et al., 2001). into collar-like structures at the base of the phagocytic cup in Septins have high affinity to End Binding 1 (EB1), a macrophages and neutrophils to enable phagocytosis (Huang protein which associates to the plus end of microtubules, et al., 2008). suggesting that septins guide microtubule polymerization in the The precise role of septins in mediating bacterial entry extending protrusion (Nölke et al., 2016). The discovery that remains to be established. Considering their function in septins mediate microtubule apical guidance can have broad compartmentalizing plasma membrane (Kusumi et al., 2012; implications in cell biology. Septins are well known to localize at Bridges and Gladfelter, 2015), septins may recruit host cell the base of the primary cilium, a membrane-delimited eukaryotic receptors, phospholipids, and/or signaling molecules to appendage also sustained by a microtubule-based scaffold that orchestrate the cytoskeletal rearrangements and membrane senses and transduces a variety of extracellular stimuli (Hu extension which underpin formation of the phagocytic cup et al., 2010). Work has shown that depletion of SEPT2 results (Barral et al., 2000; Caudron and Barral, 2009; Mostowy and in mislocalization of ciliary membrane proteins and impairs Cossart, 2011; Ostrowski et al., 2016). ciliogenesis (Hu et al., 2010). Together, these results indicate that septins play a key role in maintenance of membrane SEPTIN INTERACTIONS WITH compartmentalization and work as a membrane diffusion barrier, a function also attributed to septin rings at the mother-bud neck INTRACELLULAR BACTERIAL in S. cerevisiae (Barral et al., 2000; Takizawa et al., 2000; Caudron PATHOGENS and Barral, 2009). After phagocytosis and entry into host cells, bacteria can adopt different intracellular lifestyles. Some invasive pathogens, such as “Zipper” and “Trigger”-Mediated Entry of Chlamydia trachomatis and Legionella pneumophila, can remodel Invasive Bacteria their phagocytic compartment to establish an intracellular niche To enter non-phagocytic cells, invasive bacteria have different for persistence and replication. In contrast, other bacterial mechanisms to manipulate host signaling pathways leading to pathogens such as L. monocytogenes, S. flexneri, Mycobacterium their uptake. Some bacteria, such as L. monocytogenes and marinum, and Rickettsia spp., can escape from the phagocytic Yersinia spp., can enter host cells via a mechanism called vacuole to invade the host cell cytosol (Ray et al., 2009). “zippering” that is activated via the direct interaction between In the following section, we discuss the interactions between bacterial surface components and host cell receptors at the septins and intracellular bacteria. We focus on septin roles in plasma membrane. Other bacteria, such as S. flexneri and maintenance of the C. trachomatis intracellular niche, and also in Salmonella enterica serovar Typhimurium, enter non-phagocytic the compartmentalization of cytosolic S. flexneri for host defense. cells by a mechanism called “triggering” that is dependent on the injection of T3SS effector proteins to stimulate host cell Septin Interactions with Chlamydia membrane ruffling and engulfment via a macropinocytosis-like trachomatis process. C. trachomatis is an obligate intracellular bacterium that L. monocytogenes, a Gram-positive bacterium and foodborne replicates inside an inclusion vacuole delimited by host pathogen, can stimulate its internalization into host cells via membranes. The parasitic lifecycle of Chlamydia relies on the interactions between its surface proteins Internalin (InlA) and recruitment of actin and vimentin intermediate filaments to Internalin B (InlB) with plasma membrane proteins E-cadherin the inclusion (Kumar and Valdivia, 2008). To remodel the and Met, respectively (Pizarro-Cerdá et al., 2012). To discover cytoskeleton, C. trachomatis secretes the protease Chlamydial other proteins required for Listeria entry, proteomic analysis proteasome-like activity factor (CPAF) into the host cell was performed and identified enrichment of SEPT9 at the cytosol and increases the size/flexibility of the inclusion entry site during InlB-mediated invasion (Pizarro-Cerdá et al., to enable bacterial replication (Kumar and Valdivia, 2008). 2002). Follow-up work revealed that septin collar-like structures A recent study revealed that complexes of SEPT2-SEPT11- assembled in response to actin polymerization at sites of Listeria SEPT7-SEPT9 are also recruited to the cytosolic surface of entry (Mostowy et al., 2009b)(Figure 2C). Interestingly, SEPT2 the Chlamydia inclusion, where septins associate with actin promotes Listeria invasion while SEPT11 restricts it, suggesting filaments (Volceanov et al., 2014) (Figure 2E). Inhibition of different roles for different septins in bacterial entry (Mostowy septin assembly using forchlorfenuron reduced the expansion et al., 2009a,b, 2011a; Kühbacher et al., 2015). Septins also of inclusion vacuoles. Additionally, depletion of septins by form collar-like structures at sites of entry of “triggering” siRNA abolished the formation of septin/actin coats around bacteria, such as S. flexneri and S. Typhimurium (Mostowy the Chlamydia inclusion and restricted the extrusion of intact

Frontiers in Cell and Developmental Biology | www.frontiersin.org 4 November 2016 | Volume 4 | Article 127 Torraca and Mostowy Septin-Bacteria Interplay inclusions from host cells. The same study revealed that SEPT2 of septins and autophagy markers are tightly connected. The is a host cell substrate for CPAF, like vimentin. While the exact depletion of septins (SEPT2, SEPT7, or SEPT9) or autophagy role of SEPT2-CPAF interactions awaits investigation, it has components (p62, NDP52, ATG5, ATG6, or ATG7) abrogates been suggested that cleavage of SEPT2 by CPAF contributes to both septin cage assembly and bacterial autophagy (Mostowy dynamic remodeling of the cytoskeleton around the inclusion. et al., 2010, 2011b; Sirianni et al., 2016). These data suggest that, at least during autophagy of Shigella, septin cage assembly A Role for Septins in Actin Tail and autophagosome formation are interdependent (Mostowy Polymerization? and Cossart, 2012). It is interesting to consider that septin To evade cytosolic immunity, some bacteria that escape from cages have been observed during autophagy of S. flexneri and the phagocytic vacuole can polymerize actin for actin-based M. marinum, distinct bacterial pathogens that both polymerize motility and cell-to-cell dissemination (Welch and Way, 2013). actin via the recruitment of N-WASP (Mostowy et al., 2010, Interestingly, bacteria (including Listeria spp., Shigella spp., M. 2013; van der Vaart et al., 2014). In contrast, septin cages marinum, Burkholderia spp., and Rickettsia spp.) have evolved have not been observed for Listeria spp. (Mostowy et al., different mechanisms to polymerize actin tails (Gouin et al., 2010), partly because cytosolic Listeria polymerize actin tails 2005; Welch and Way, 2013). Just as septins are recruited to via ActA, a bacterial effector that mimics host cell N-WASP actin polymerization at the plasma membrane, septins are also and prevents the ubiquitination and recognition of bacteria recruited to some actin-polymerizing bacteria in the cytosol as by autophagy (Yoshikawa et al., 2009; Mostowy et al., 2011b). ring-like structures (Mostowy et al., 2010)(Figure 2F). Although How exactly septins and autophagy are recruited to bacteria work using septin-depleted cells failed to reveal any role for remains to be established. Recent work in S. cerevisiae has shown septins in the functionality of actin tails (Mostowy et al., 2010), that septins are involved in the early stages of autophagosome it is possible that septins impact actin tails in aspects difficult to formation during starvation (Barve et al., 2016). Collectively, detect using transient depletion techniques in tissue culture cells. these data suggest that septins have a fundamental role in Given that both actin-based motility and septin assembly can multiple pathways of autophagy, including starvation-induced be reconstituted using purified proteins in cell-free conditions autophagy and bacterial autophagy. (Loisel et al., 1999; Bridges and Gladfelter, 2016), it seems likely To better understand the molecules and events required for that biochemical reconstitution systems will be of great value to septin cage assembly, proteomic analysis of the Shigella-septin elucidate whether septins have a role in the functionality of actin cage was performed (Sirianni et al., 2016). From this approach, 56 tails. host proteins were identified to interact with septins in Shigella- infected conditions, including p62 and the ATG8 family member Septin Interplay with Bacterial Autophagy Light Chain 3 B (LC3B). Surprisingly, proteomics also revealed Septins can recognize some bacterial pathogens that escape to that 21.4% of septin cage-associated proteins were annotated as the host cell cytosol and polymerize actin, forming cage-like exclusively mitochondrial. Indeed, high-resolution microscopy structures to entrap bacteria and prevent actin tail formation showed that mitochondria support septin assembly into the cages (Mostowy et al., 2010) (Figure 2G). Indeed, the frequency of that entrap Shigella for autophagy. A role for mitochondria actin tails is significantly enhanced in septin-depleted cells in septin cage assembly was confirmed by depletion of factors infected with bacteria normally recognized by septin cages. implicated in mitochondria fission and fusion (Sirianni et al., Recent work has also shown that septin cages assemble to restrict 2016). The depletion of Dynamin Related Protein 1 (DRP1), bacterial replication (Sirianni et al., 2016). Septin cages have which elongates mitochondria and increases their availability as been observed for both S. flexneri and M. marinum during a membrane source for septin assembly, significantly increased infection of tissue culture cells in vitro and during infection of septin caging. Conversely, the depletion of Mitofusin 1 (MFN1), zebrafish in vivo (Mostowy et al., 2010, 2013; Sirianni et al., which fragments mitochondria and limits their availability as 2016). Collectively, these observations highlight septin caging a membrane source for septin assembly, significantly reduced of cytosolic bacteria as an evolutionarily-conserved assembly septin caging. Shigella invasion is well known to induce valuable for host defense (Mostowy, 2014; Mostowy and Shenoy, mitochondria fragmentation (Carneiro et al., 2009; Lum and 2015). Morona, 2014). Remarkably, fragmentation of mitochondria Shigella entrapped in septin cages are also targeted to enables Shigella to escape from septin cages and autophagy autophagy (Mostowy et al., 2010; Sirianni et al., 2016) recognition (Sirianni et al., 2016). L. monocytogenes, which (Figure 2G). Autophagy is a membrane trafficking process avoids septin caging, can induce fragmentation of mitochondria that can redirect cytosolic material, including bacteria, to lytic via Listeriolysin O (LLO), a pore-forming toxin (Stavru et al., compartments (Levine et al., 2011; Mostowy, 2014). A group of 2011). Together, these observations suggest that fragmentation autophagy-related (ATG) proteins constitute the core machinery of mitochondria may be a general mechanism for bacterial responsible for the formation, elongation, and maturation pathogens to escape from cell-autonomous immunity (Sirianni of the autophagosome. In the case of bacterial autophagy, et al., 2016). ubiquitination and its recognition by a set of ubiquitin-binding Important studies have shown that septins recognize micron autophagy receptors (e.g., p62/SQSTM1, NDP52/CALCOCO2), scale curvature at the plasma membrane, and that septin can direct the ATG machinery around bacterial cargo (Khaminets assembly is membrane-facilitated (Tanaka-Takiguchi et al., et al., 2016; Maculins et al., 2016). Strikingly, the recruitment 2009; Bridges and Gladfelter, 2015; Bridges et al., 2016). In

Frontiers in Cell and Developmental Biology | www.frontiersin.org 5 November 2016 | Volume 4 | Article 127 Torraca and Mostowy Septin-Bacteria Interplay agreement with this, septin assemblies are closely associated HeLa). Considering the emerging roles for the host cytoskeleton with invaginations and protrusions of the plasma membrane in cell-autonomous immunity (Mostowy and Shenoy, 2015), including the phagocytic cup, the cleavage furrow, and the future work should address septin function in immune cells. base of dendritic spines and cilia (Bridges et al., 2016; Lobato- More insight into septin roles in host defense can also come from Márquez and Mostowy, 2016). In contrast, septin association the use of animal models. Indeed, septin-bacteria interactions can with sources of membrane in the cytosol remains to be be studied using zebrafish infection models innovated to study established. New studies have shown that septins interact the cell biology of infection in vivo (Mostowy et al., 2013; Willis with mitochondria and enable mitochondrial fission, in a et al., 2016). We propose that use of in vivo systems, including process called mitokinesis (Pagliuso et al., 2016; Sirianni et al., the zebrafish and mouse, will be essential to illuminate a more 2016). Strikingly, septin function at the plasma membrane complete understanding of septin biology during host defense (e.g., phagocytosis, cytokinesis) and in the cytosol (e.g., septin against bacterial infection. caging, mitokinesis) all involve actin and non-muscle myosin II (Mostowy and Cossart, 2012; Sirianni et al., 2016). Next, it will be AUTHOR CONTRIBUTIONS important to investigate different sources of cytosolic membrane (e.g., mitochondria, ER), and their precise role in septin assembly. VT and SM jointly wrote the manuscript.

CONCLUSIONS ACKNOWLEDGMENTS

Investigation into the cell biology of bacterial infection has VT is supported by a Marie Skłodowska-Curie Individual significantly improved our understanding of both infection and Fellowship (H2020-MSCA-IF-2015 – 700088). Work in SM cytoskeleton biology (Haglund and Welch, 2011). For example, laboratory is supported by a Wellcome Trust Research Career the study of actin-tail formation by cytosolic bacteria has enabled Development Fellowship (WT097411MA) and the Lister major discoveries underlying bacterial pathogenesis and actin Institute of Preventive Medicine. biology (Welch and Way, 2013). In this review, we have discussed several examples of how investigation of septin assembly in SUPPLEMENTARY MATERIAL response to bacterial invasion is discovering novel aspects of septin biology and cell-autonomous immunity. Our current The Supplementary Material for this article can be found knowledge of septin function during infection mostly derives online at: http://journal.frontiersin.org/article/10.3389/fcell. from work performed in vitro using non-phagocytic cells (e.g., 2016.00127/full#supplementary-material

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inclusion and are required for normal release of the inclusion by extrusion. autophagic recognition. Nat. Cell Biol. 11, 1233–1240. doi: 10.1038/ MBio 5, e01802–01814. doi: 10.1128/mBio.01802-14 ncb1967 Weiner, A., Mellouk, N., Lopez-Montero, N., Chang, Y. Y., Souque, C., Schmitt, C., et al. (2016). Macropinosomes are key players in early shigella invasion Conflict of Interest Statement: The authors declare that the research was and vacuolar escape in epithelial cells. PLoS Pathog. 12:e1005602. doi: conducted in the absence of any commercial or financial relationships that could 10.1371/journal.ppat.1005602 be construed as a potential conflict of interest. Welch, M. D., and Way, M. (2013). Arp2/3-mediated actin-based motility: a tail of pathogen abuse. Cell Host Microbe 14, 242–255. doi: Copyright © 2016 Torraca and Mostowy. This is an open-access article distributed 10.1016/j.chom.2013.08.011 under the terms of the Creative Commons Attribution License (CC BY). The Willis, A., Mazon-Moya, M., and Mostowy, S. (2016). Investigation of septin use, distribution or reproduction in other forums is permitted, provided the biology in vivo using zebrafish. Methods Cell Biol. 136, 221–241. doi: original author(s) or licensor are credited and that the original publication in 10.1016/bs.mcb.2016.03.019 this journal is cited, in accordance with accepted academic practice. No use, Yoshikawa, Y., Ogawa, M., Hain, T., Yoshida, M., Fukumatsu, M., Kim, distribution or reproduction is permitted which does not comply with these M., et al. (2009). Listeria monocytogenes ActA-mediated escape from terms.

Frontiers in Cell and Developmental Biology | www.frontiersin.org 8 November 2016 | Volume 4 | Article 127